Create a new entity. Simple entities can be constructed by passing a name,
value to the constructor; this creates a generic, plain entity reference.
For anything more complicated, you have to pass a Source to the constructor with the entity definiton,
or use the accessor methods. WARNING: There is no validation
of entity state except when the entity is read from a stream. If you start
poking around with the accessors, you can easily create a non-conformant Entity. The best thing to do is dump the stupid
DTDs and use XMLSchema instead.

Returns the value of this entity. At the moment, only internal entities
are processed. If the value contains internal references (IE, %blah;),
those are replaced with their values. IE, if the doctype contains: